The field of this invention is the electrochemical determination of analyte in biological fluids, particularly the electrochemical determination of the adequacy of the volume of the biological fluid sample to be tested for analyte concentration.
Analyte concentration determination in biological fluids, e.g., blood or blood-derived products such as plasma, is of ever increasing importance to today""s society. Such assays find use in a variety of applications and settings, including clinical laboratory testing, home testing, etc., where the results of such testing play a prominent role in the diagnosis and management of a variety of disease conditions. Common analytes of interest include glucose for diabetes management, cholesterol for monitoring cardiovascular conditions, and the like. In response to this growing importance of analyte concentration detection, a variety of analyte detection protocols and devices for both clinical and home use have been developed.
One type of method that is employed for analyte detection is an electrochemical-based method. In such methods, an aqueous liquid sample is placed into a reaction zone in an electrochemical cell made up of at least two electrodes, i.e., a counter/reference electrode and a working electrode, where the electrodes have an impedance which renders them suitable for amperometric measurement. The component to be analyzed, i.e., analyte, is allowed to react directly with an electrode, or directly or indirectly with a redox reagent to form an oxidisable (or reducible) substance in an amount corresponding to the concentration of the analyte. The quantity of the oxidisable (or reducible) substance present is then estimated electrochemically and related to the amount of analyte present in the initial sample.
Commonly, the electrochemical cell is in the form of a disposable test strip on which the biological sample is deposited and which is receivable within a meter by which the electrochemical analyte concentration measurement is made. Examples of assay systems that employ these types of test strips, often referred to as biosensors, and meters may be found in U.S. Pat. Nos. 5,942,102, 6,174,420 B1 and 6,179,979 B1, the disclosures of which are herein incorporated by reference. With these systems, determination of the concentration of an analyte in a biological sample first involves obtaining a biological sample and bringing that sample into contact with a reaction area of the test strip so that the biological sample, and more particularly the analyte of interest or derivative thereof, may react with the chemistry, e.g., the testing reagent(s), associated with the reaction area. In order to obtain an accurate measurement of the particular analyte(s) of interest, a minimum sample volume must be applied to the reaction area. It is not uncommon for an inadequate amount of sample volume to be provided, often due to user error or patient inexperience or misjudgment. Inaccurate measurements can result in a misdiagnosis or improper treatment, such as administering an inappropriate dosage of a drug, patient non-compliance, etc. Such can result in serious and even life-threatening consequences for those whose lives depend on frequent monitoring of an analyte in their body, for example, diabetics.
One approach to ensuring an adequate biological sample volume is to over-saturate or use a greater volume of sampled fluid than is necessary to fill the reaction area of the test strip. A disadvantage of using an unnecessarily large volume of a sampled fluid, a blood sample in particular, is the need to draw a greater volume of blood sample from the patient. This requires use of a blood sample volume, which is rather large, thus necessitating use of a larger diameter needle and/or deeper penetration into the skin. These factors can increase the discomfort and pain felt by the patient, and may be difficult to achieve for those individuals whose capillary blood does not readily express. As this sampling process may be repeated frequently within a single day, for many diabetics, for example, an increase in pain quickly becomes less tolerable or intolerable all together.
Some analyte detection biosensors have been developed to provide visual confirmation of the adequacy of sample volume, however, this feature does not exclude potential error by the patient in judging the adequacy of the sample""s volume, e.g., diabetics may experience deteriorated vision. Certain other analyte determination biosensors do provide user-independent means for determining the adequacy of the sample volume. Examples of such biosensors are disclosed in U.S. Pat. Nos. 5,628,890 and 5,650,062 and PCT Patent Application Publication No. WO 99/32881 (PCT Patent Application No. PCT/US98/27203). In particular, the ""881 publication describes an electrochemical glucose monitoring system which attempts to determine the adequacy of a volume of sample applied to a biosensor by applying a low-level AC voltage signal (without a DC voltage offset) at a known frequency to the biosensor and then measuring both the real component and the imaginary component of the resulting impedance. These impedance values are then compared to a look-up table in the microprocessor""s program memory. The accuracy of this method may be additionally questionable considering that this system is dependent on blood hematocrit levels and environmental temperature variations.
Another disadvantage of the technique disclosed in the ""881 publication is that the analyte measurement test must be aborted if the sample volume is determined to be inadequate, i.e., a xe2x80x9cgo-no-goxe2x80x9d situation. This results in the need to take yet another sample from the patient which, as mentioned above, is inconvenient and may be very painful to the patient, likely resulting in patient non-compliance in his or her medication regime. Additionally, the test must be repeated resulting in the waste of test strips and increasing the cost of the procedure.
As such, there is continued interest in the identification of new techniques for accurately and precisely measuring the adequacy of the volume of the sample used for electrochemical analyte concentration determination. Of particular interest would be the development of methods that can very accurately and expeditiously determine the adequacy of the volume of sample. It would be additionally beneficial to develop such a sample volume adequacy determination technique in which a determination that a sample volume is inadequate does not require abortion of the analyte concentration measurement test. Ideally, this technique would compensate for the less than optimal sample volume and provide an accurate analyte concentration measurement without having to provide a new sample or conduct a new test.
The present invention provides methods and systems for measuring the volume of biological sample and determining whether such volume is adequate to produce an accurate measurement of at least one selected characteristic of the biological sample, such as the concentration of an analyte contained therein. Certain such methods and systems provide the additional function of compensating for a sample volume determined to be less than adequate in order to proceed with a measurement procedure.
The present invention is employed with a biosensor, such as an electrochemical test strip to which the sample volume of biological solution is deposited, and a meter configured to receive such test strip and to measure the concentration of selected analytes within the biological sample. The electrochemical test strip, as will be more fully described below, includes an electrochemical cell comprised of opposing electrodes between which a reaction zone is defined for receiving the biological sample, wherein the reaction zone has a defined thickness and volume.
When sufficient voltage is applied to an electrochemical cell, both double layer charging and electrochemical reaction will occur. As a consequence, charge flows to the electrodes of an electrical cell. The electrode-solution interface is analogous to that of a capacitor. The ratio of this charge to the voltage determines the capacitance of the electrode-solution interface. Since the total charge is due to the charging of the double layer and to the electrochemical reaction, two distinct capacitance components, Cdl and Cs, respectively, contribute to the total or equivalent capacitance of the cell (see Bard, A. J. and Faulkner, L. R., Electrochemical Methods, 1980).
The inventors have discovered that the equivalent capacitance of an electrochemical cell is the most relevant factor in precisely determining sample volume, as the equivalent cell capacitance is linearly proportional to the amount of surface area of the cell electrodes in contact with the sample (the xe2x80x9ccovered cell areaxe2x80x9d), and thus, is linearly proportional to the volume of the sample within the cell, i.e., between the electrodes. The inventors have also discovered that the equivalent resistance of the electrochemical cell is additionally relevant in precisely determining sample volume, as the equivalent cell resistance is inversely proportional to the covered cell area, and thus, is inversely proportional to the sample volume.
Thus, a feature of the present invention is to derive such covered cell area and the corresponding sample volume from the equivalent cell capacitance or from both the equivalent cell capacitance and the equivalent cell resistance.
Another feature of the present invention is to control certain other factors (e.g., the thickness of the cell, the concentration of ionic species, etc.) that may interfere with accurately measuring sample volume in order that the value of the equivalent cell capacitance is independent and unaffected by the glucose concentration and blood hematocrit levels within the sample, the environmental temperature, particularities of the blood donor and other commonly interfering components of blood.
Yet another feature of the present invention is to provide the additional function of compensating for a sample volume determined to be less than adequate in order to proceed with an accurate analyte concentration measurement.
Accordingly, the present invention provides methods for determining the adequacy of the volume of a biological sample to be used for determining the concentration of one or more selected analytes within the donor sample, which achieves these objectives and provides these features.
In certain embodiments of the subject methods, an alternating current voltage (AC voltage) of low amplitude and having a selected frequency is applied to a biosensor containing the biological sample to be tested, thereby charging the biosensor. Optionally, a direct current voltage (DC voltage) may be applied simultaneously along with the AC voltage in order to increase the rate at which the capacitance of the biosensor becomes stabilized. The resulting alternating current generated from such charging is then measured, and the equivalent cell capacitance of the biosensor is then determined from the resulting alternating current. The equivalent cell capacitance is then used to determine the amount of surface area of the biosensor in contact with the sample solution, which surface area is then used to derive the volume of the sample within the biosensor. Upon a determination that the sample volume is adequate to make an accurate analyte concentration measurement, such analyte concentration is measured. On the other hand, if it is determined that the sample volume is inadequate, the subject methods may further include compensating for such inadequate sample volume during the analyte concentration measurement process. Inadequate volume compensation involves determining the necessary compensation factor which includes, at least in part, determining the ratio of the equivalent cell capacitance of the biosensor containing the actual sample volume to the cell capacitance of the biosensor when its entire available volume is filled.
The present invention also includes systems for carrying out the subject methods. The subject systems include electronic components and/or circuitry intended to be used with and electronically coupled to a biosensor, e.g., an electrochemical measurement cell in the form of, e.g., a disposable test strip, into which the sampled solution to be tested is deposited or is drawn by a capillary action. Most typically, such electronic circuitry is incorporated into a meter or other automated device configured to receive and operatively engage with such electrochemical cell, e.g., a disposable test strip, and to measure one or more physical or chemical characteristics of a biological sample held within the electrochemical cell. Most typically, such characteristics include the concentration of one or more target analytes within the biological sample. Such electronic circuitry may comprise discrete electronic components, e.g., a voltage supply, and/or integrated circuits having multiple circuit elements and/or semiconductor devices, e.g., a microprocessor suitably programmed to execute certain steps or functions of the subject methods based on certain signal or data inputs received from the electrochemical cell.
In certain embodiments, the systems of the present invention include such electronic circuitry and such an automated measurement device or meter, as just described, wherein the electronic circuitry is completely structurally and functionally integral with the automated measurement device.
While the subject methods and systems may be used to determine the sample volume of different biological samples, such as urine, tears, saliva, and the like, they are particularly suited for use in determining the sample volume of blood or blood fractions and the like. Furthermore, while the subject systems and methods for determining the sample volume in preparation for measuring a variety of physical and chemical characteristics of the sample, they are particularly useful in preparation for measuring the concentration of selected analytes within the sample.
These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the methods and systems of the present invention which are more fully described below.